Y-capacitance fault current discharge compensation for HVDC systems

ABSTRACT

A fuel cell system or HVDC power source includes a fuel cell or DC power source having a high voltage direct current (HVDC) bus including y-capacitors, which connect the HVDC bus terminals to chassis or safety ground, and a fault current discharge compensation circuit interconnected with the HVDC bus. A monitoring circuit monitors a fault discharge current of the Y-cap circuit and generates a fault signal when the fault current occurs based on a rate of change of a voltage of the Y-cap circuit. A switching circuit redirects the majority of the fault discharge current.

FIELD OF THE INVENTION

The present invention relates to fuel cells or other automotive HVDC sources, and more particularly to a y-capacitor discharge fault current compensation system for fuel cell stacks.

BACKGROUND OF THE INVENTION

Fuel cell stacks operate at relatively high voltage levels and higher temperatures. The fuel cell system includes a high voltage direct current (HVDC) bus that provides power from the fuel cell stacks to a load. The HVDC bus is subject to electromagnetic interference (EMI), which detrimentally effects the performance of the fuel cell system. Y-capacitors (Y-caps) are incorporated to protect the HVDC bus from EMI. The Y-caps bridge the positive and/or negative DC to chassis or safety ground.

In the event of a fault contact, the Y-caps discharge through the resistance of the fault contact. For example, when a person touches either the positive or negative nodes of the HVDC bus and chassis or safety ground, the Y-caps discharge through the body of the person. As a result, the capacitance of the Y-caps must be limited to ensure that the energy released in the event of a fault contact is within safe limits. Because the Y-caps are limited, the ability to protect against EMI is correspondingly limited.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a fuel cell system that includes a fuel cell having a high voltage direct current (HVDC) bus and a fault current discharge compensation circuit interconnected with the HVDC bus. The HVDC bus includes a Y-capacitance (Y-cap) circuit that bridges the positive and/or negative HVDC terminals to chassis or safety ground. A monitoring circuit monitors a fault discharge current of the Y-cap circuit and generates a fault signal when the fault current occurs. A switching circuit redirects the fault discharge current based on a rate of change of a voltage of the Y-cap circuit.

In one feature, the Y-cap circuit bridges positive and negative terminals of the HVDC bus to chassis or safety ground.

In another feature, the monitoring circuit generates the fault signal based on the rate of change of the output voltage of the HVDC bus terminals with respect to chassis or safety ground.

In another feature, the fault signal is based on the fault discharge current.

In another feature, the switching circuit includes an operational amplifier that generates an output signal. A first switch selectively enables an alternate path from HVDC negative to ground for the fault discharge current based on the output signal. A second switch selectively enables an alternate path from HVDC positive to ground for the fault discharge current based on the output signal. When a fault condition is generated at a positive terminal of the HVDC bus, the op-amp signals the second switch to enable the alternate path. When a fault condition is generated at a negative terminal of the HVDC bus, the op-amp signals the first switch to enable the alternate path.

Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 is an electrical schematic of a high voltage (HV) bus including Y-capacitors;

FIG. 2 is an electrical schematic of the HV bus incorporating a Y-capacitor (Y-cap) discharge compensation circuit according to the present invention;

FIG. 3 is a graph illustrating fault discharge currents according to the present invention; and

FIG. 4 is an electrical schematic of the HV bus incorporating an active isolation circuit according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference designations will be used in the drawings to identify similar elements.

Referring now to FIG. 1, a fuel cell system 10 includes a high voltage direct current (HVDC) power bus 12 and a fuel cell stack 14. The fuel cell stack 14 is represented as two voltage sources V₁ and V₂. Exemplary values for V₁ and V₂ are 200V, although other values may be used. Assuming 200V for V₁ and V₂, the total voltage across the fuel cell stack 14 is 400V. The fuel cell stack 14 includes coolant flowing through manifolds. Since the coolant can be conductive (not a perfect electrical insulator), the coolant forms resistive paths from the fuel cell through the coolant tubes towards grounded metallic parts of the coolant system (e.g. vehicle front radiator). The coolant inlet/exit is indicated as parallel resistors R_(c). Exemplary values for the resistors R_(c) are 20 kΩ each or 10 kΩ total.

The HVDC power bus 12 includes positive and negative nodes (HV+ and HV−, respectively) and a capacitor circuit 16. Given the exemplary values of V₁ and V₂ and assuming the voltage balance is symmetrical, HV+ is at +200V and HV− is at −200V. The capacitor circuit includes capacitors C₁, C₂ and C₃. C1 is called X-capacitor and bridges HV+ and HV−. C2 and C3 are called Y-capacitors and bridge HV+ to chassis or safety ground or HV− to chassis or safety ground. Capacitors can be distributed across multiple electric devices connected to the HVDC bus and are represented as lumped single capacitors here. Exemplary values for C₁, C₂ and C₃ are 3000 μF, 5 μF and 5 μF, respectively. The Y-capacitors C2,C3 protects the HVDC power bus 12 from electromagnetic interference (EMI).

A typical fault contact, for example a human body, is indicated as a fault resistance R_(FAULT). Although the fault contact is shown at HV+, the fault contact can also occur at HV− or at any intermediate voltage. An exemplary value for R_(FAULT) is 1 kΩ. As a result of the fault contact, a discharge current causes the Y-cap circuit 16 to discharge through R_(FAULT) to ground. The energy in the Y-cap circuit that is dissipated during the fault contact is equal to ½CV². As shown in FIG. 3, which is discussed in further detail below, the typical discharge current immediately peaks upon fault contact and then gradually decreases to under 25 mA, given the exemplary values provided herein. The area beneath the typical discharge current curve indicates the energy that is dissipated through R_(FAULT) (e.g., human body).

Referring now to FIG. 2, a fuel cell system 20 includes a high voltage direct current (HVDC) power bus 22 and a fuel cell stack 24. The fuel cell stack 24 is represented as two voltage sources V₁ and V₂. Exemplary values for V₁ and V₂ are 200V, although other values may be used. Assuming 200V for V₁ and V₂, the total voltage across the fuel cell stack 24 is 400V. The fuel cell stack. 24 includes coolant flowing through manifolds, which is indicated as parallel resistors R₁ and R₄. Exemplary values for R₁ and R₄ are 22 kΩ and 18 kΩ, respectively. The coolant is provided by a coolant system 26 as indicated by parallel resistors R₉ and R₈. Exemplary values for R₉ and R₈ are 10 kΩ each. R₉ and R₈ are in respective series connection with R₁ and R₄. The coolant represented by R8 and R9 are in contact with chassis or safety ground through metallic coolant loop members (e.g. radiator).

The HVDC power bus 22 includes positive and negative nodes (HV+ and HV−, respectively) and a capacitor circuit 28. Given the exemplary values of V₁ and V₂ and assuming that the voltage balance is symmetrical, HV+ is at +200V and HV− is at −200V. The cap circuit 28 includes capacitors C₈, C₁ and C₂. Exemplary values for C₈, C₁ and C₂ are 3000 μF, 5 μF and 5 μF, respectively. The cap-circuit 28 protects the HVDC power bus from electromagnetic interference (EMI). The Y-capacitors C2,C3 bridges the HVDC power bus to a vehicle chassis (not shown) or safety ground.

A Y-cap discharge compensation circuit 29 bridges the HVDC power bus 22 and includes a monitoring circuit 30 and a switching circuit 32. The monitoring circuit 30 includes capacitors C₁₁, C₁₂ and C₁₃ and resistors R_(Y-CAP), R₁₈, R₁₉, R₂₁, and R₂₂. Exemplary values for C₁₁, C₁₂ and C₁₃ include 1 μF each. An exemplary value for R_(Y-CAP) includes 100 Ω and exemplary values for R₁₈, R₁₉, R₂₁ and R₂₂ include 5 kΩ each.

The switching circuit 32 includes an operational amplifier (op-amp) 34, a first n-Channel MOSFET transistor switch S₁ and a second p-Channel MOSFET transistor S₂. The op-amp 34 includes a positive input 36 that is connected to ground. An output 38 is connected to S₁ and S₂. A negative input 40 is connected to the monitoring circuit and the output through a capacitor C₇ and a resistor R₇. S₁ includes a gate input 42 that is connected to the op-amp output 38. An input 46 (Drain) is connected to HV− through a resistor R₁₇ and an output 48 (Source) is connected to ground through a resistor R_(INJ). S₂ includes a gate input 50 that is connected to the op-amp output 38. An input (Drain) 54 is connected to HV+ through a resistor R₁₆ and an output 56 (Source) is connected to ground through the resistor R_(INJ). Exemplary values for R₁₆ and R₁₇ include 50 Ω each and an exemplary value for R_(INJ) includes 10 Ω. S₁ and S₂ function as switches. When in a conductive state, S₁ and S₂ provide an alternate current path from the HVDC bus terminals to ground through R_(INJ) and R16 or R 17.

In operation, the monitoring circuit 30 provides current to the switching circuit 32 indicating a discharge current of the Y-capacitors C2,C3 circuit 28. More particularly, the monitoring circuit 30 monitors the rate of change of voltage (dV/dt) of the HVDC bus terminals with respect to chassis or safety ground. If dV/dt of the HVDC bus terminals is greater than a threshold level, a fault discharge current situation is indicated. That is to say, the Y-capacitors C2 or C3 are being caused to discharge by a fault contact such as a person touching either HV+, HV− or any intermediate voltage point.

The op-amp 34 receives the current signal from the monitoring circuit 30. More particularly, the dV/dt signal is generated by the differentiating capacitor-resistor network that includes R_(Y-CAP) and C₁₂. The dV/dt signal is filtered and smoothed by R21 and C₁₃. The filtered signal causes the output 38 of the Op-Amp to change to positive or negative depending on the sign of dV/dt, which depends on the fault location being on the positive or negative HVDC bus terminal. If the OpAmp output exceeds the turn on gate threshold voltage of the MOSFET switches S1 (e.g. −5V) or S2 (e.g. +5V), it causes S1 or S2 to turn on, which redirects the main fault discharge current path. For example, in the event of a fault at HV+, as illustrated in FIG. 2, the op-amp output closes S₂ to create a discharge path to ground through R16 and R_(INJ). As a result, the energy of the Y-cap circuit 28 is dissipated mainly through R16 and R_(INJ) instead of through R_(FAULT). Similarly, in the event of a fault at HV−, the op-amp output closes S₁ to create a discharge path to ground through R17 and R_(INJ).

Referring now to FIG. 3, a graph illustrates Y-cap fault discharge currents according to the present invention. Typical discharge currents for conventional circuits are illustrated by the dashed lines. The discharge current for the discharge compensation circuit 29 of the present invention is illustrated by the solid line. The discharge current drops more rapidly. Additionally, the area under each of the curves indicates the amount of energy dissipated through R_(FAULT). A significantly decreased amount of energy is dissipated through R_(FAULT) using the discharge compensation circuit 29.

Referring now to FIG. 4, the fuel cell system 20 includes an active isolation circuit 60. The active isolation circuit includes ground fault current sensors 62, 64 that are associated with the coolant. The fault sensors 62, 64 are connected to the inverting input 40 of the op-amp 34 and ground through resistors R_(S1) and R_(S2), respectively. The fault sensors 62, 64 measure net fault current flowing through all coolant resistant paths of the fuel cell system 20. Although the fuel cell system 20 of FIG. 4 is shown to include both the Y-cap discharge compensation circuit 29 and the active isolation circuit 60 together, the function of the active isolation circuit 60 can be achieved using the active isolation circuit 60 and the switching circuit 32 alone.

In the event of a sufficient fault current through the coolant resistance paths, the active isolation circuit 60 signals the switching circuit 32 to provide a discharge path to ground. For example, when a sufficient negative fault current is detected by the fault sensor 64 or 62, the op-amp output closes S₂ to create a discharge path to ground through R16+R_(INJ). As a result, the fault current is forced towards 0 mA. Similarly, when a sufficient positive fault current is detected by the fault sensor 62 or 64, the op-amp output closes S1 to create a discharge path to ground through R17 and R_(INJ), again resulting in the fault current being forced towards 0 mA.

The active isolation circuit 29 supports a fuel cell stack coolant scheme that includes a low conductivity coolant entering and exiting the fuel cell stack 24. Furthermore, implementation of the active isolation circuit 29 requires the use of isolated or non-conductive coolant manifolds or non-conductive coolant entrance and exit areas to form a high resistance path upstream and downstream of the fault sensors 62, 64.

Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the current invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims. 

1. An apparatus, comprising: a high voltage direct current (HVDC) source having an HVDC bus; a capacitor circuit incorporating y-capacitors which bridge terminals of said HVDC bus to chassis or safety ground; a fault current discharge compensation circuit interconnected with- said HVDC bus and comprising: a monitoring circuit that monitors a fault discharge current of said Y-cap circuit and that generates a fault signal when said fault current occurs; and a switching circuit that redirects said fault discharge current based on a rate of change of a voltage of said Y-cap circuit with respect to chassis or safety ground.
 2. The apparatus of claim 1 wherein said monitoring circuit generates said fault signal based on said rate of change of said output voltage of the positive or negative HVDC bus with respect to chassis or safety ground.
 3. The apparatus of claim 1 wherein said fault signal is based on said fault discharge current.
 4. The apparatus of claim 3 wherein said switching circuit comprises: an operational amplifier that generates an output signal; and a first switch that selectively enables an alternate path to ground for said fault discharge current based on said output signal; and a second switch that selectively enables an alternate path to ground for said fault discharge current based on said output signal.
 5. The apparatus of claim 3 wherein when a fault condition is generated at a positive terminal of said HVDC bus, said op-amp signals said second switch to enable said alternate path.
 6. The apparatus of claim 3 wherein when a fault condition is generated at a negative terminal of said HVDC bus, said op-amp signals said first switch to enable said alternate path.
 7. A fault current discharge compensation circuit for a high voltage direct current (HVDC) system including a HVDC bus with a Y-capacitance (Y-cap) circuit that communicates with said HVDC bus, comprising: a monitoring circuit that monitors said Y-cap circuit for an fault current and that generates a fault signal when said fault current occurs; and a switching circuit that redirects said fault discharge current when said monitoring circuit generates said fault signal.
 8. The fault discharge compensation circuit of claim 7 wherein said monitoring circuit generates said fault signal based on a discharge rate of said Y-cap circuit.
 9. The fault discharge compensation circuit of claim 7 wherein said fault signal is based on said-fault discharge current.
 10. The fault discharge compensation circuit of claim 7 wherein said switching circuit comprises: an operational amplifier that generates an output signal; and a switch that selectively enables an alternate path to ground for said discharge current based on said output signal.
 11. The fault discharge compensation circuit of claim 10 wherein said switch includes MOSFET transistors that enables said alternate path when in an on mode.
 12. A fault current discharge compensation circuit interconnected with a high voltage direct current (HVDC) bus of a fuel cell system with a Y-capacitance (Y-cap) circuit that communicates with said HVDC bus, comprising: a monitoring circuit that monitors an fault discharge current of said Y-cap circuit and that generates a fault signal when said fault current occurs; and a switching circuit that redirects said fault discharge current based on a rate of change of a voltage of said Y-cap circuit.
 13. The fault discharge compensation circuit of claim 12 wherein said monitoring circuit generates said fault signal based on said rate of change of said voltage.
 14. The fault discharge compensation circuit of claim 12 wherein said fault signal is based on said fault discharge current.
 15. The fault discharge compensation circuit of claim 12 wherein said switching circuit comprises: an operational amplifier that generates an output signal; and a first switch that selectively enables an alternate path to ground for said fault discharge current based on said output signal; and a second switch that selectively enables an alternate path to ground for said fault discharge current based on said output signal.
 16. The fault discharge compensation circuit of claim 15 wherein when a fault condition is generated at a positive terminal of said HVDC bus said op-amp signals said second switch to enable said alternate path.
 17. The fault discharge compensation circuit of claim 15 wherein when a fault condition is generated at a negative terminal of said HVDC bus said op-amp signals said first switch to enable said alternate path. 